As illustrated in FIG. 1, conventional xenon lamps generally have a construction in which electrodes 1 and 1' such as thoriated tungsten electrodes are sealed in the ends of a quartz glass tube 2, the inside of which is charged with xenon gas, and the electrodes are connected to an electric circuit for causing a discharge through the xenon for causing the emission of light. Upon discharge, the electrodes are heated and the temperature of the tip ends, thereof becomes high, causing a small quantity of the metal of the electrodes to evaporate and then condense on and adhere throughout the inside of the tube 2. This deposit blackens the tube. The blackened tube absorbs light and the quantity of transmitted light decreases. For example, it has been found that after a lighting period of 100 hours, the quantity of light emitted from the tube decreases by about 10% to 30%. Consequently, if a xenon lamp having such a construction is used as a light source in a light fastness tester which required a stable or constant quantity of light, correct test resuls cannot be obtained.
As means for preventing this blackening, there has been proposed an arrangement in which a ring magnet is provided around the outside of the tube of the xenon lamp near the electrodes, as in U.S. Pat. No. 3,991,336 entitled "Magnetized xenon lamp". This arrangement is indeed effective to prevent the occurrence of blackening in the portion of the tube from which most of the light is emitted, but it is disadvantageous in that the tube wall near the ring-shaped magnet blackens to an extreme degree and eventually becomes whitely turbid and devitrified, thus becoming liable to undergo fissure and breakage.